On analysing the role of probiotics in oysters, the team of scientists found out that the Vibrio bacteria OY15 works by stimulating the specimen’s immune system. However, they initially expected to see probiotics working by competitive exclusion of potential pathogens.

Therefore, the researchers concluded that the OY15 genome revealed that it lacks the genes that cause pathogenicity.

"We think it's probably a harmless former pathogen that nevertheless is recognized by the shellfish immune system as a potential threat, and that up-regulates the immune system," pointed out Gary Wikfors, a NOAA Fisheries scientist involved in the project.

To find probiotic strains, the scientists collected bacteria that occur naturally in the digestive glands of healthy adult Eastern oysters. They then grew those candidate strains in petri dishes along with a pathogenic strain of Vibrio bacteria that had caused a disease outbreak a few years before. The strains that were most effective at inhibiting growth of the pathogen in a petri dish then advanced to the next stage of testing: the guts of live oyster larvae.

The 15th strain they tested, dubbed OY15 by scientists, was the most effective. "We see higher survival of larvae that have been given OY15," said Diane Kapareiko, the NOAA Fisheries scientist who led the effort. "Better survival means more seed to sell."

Although probiotics are already used at salmon and shrimp farms, this is the first time they have been developed for bivalve shellfish and it is highly welcomed by oyster seed producers.

At the Oyster Seed Holding Company in Matthews, Virginia, near the mouth of the Chesapeake Bay, oyster seed is produced and the sold to East Coast oyster farmers. Oyster seed are 2-week-old oysters fed on a a healthy diet to make sure it survives. During the cooler months, the firm’s yield is around 40 per cent, but in summer, when bacteria that can harm the delicate larvae proliferate in the warmer water, its yield is half that.

In some parts of the world, antibiotics are used to control bacterial disease when producing seed at shellfish hatcheries. But antibiotics, which can give rise to resistant strains of pathogens, are not approved for use in growing oysters or other bivalves in the United States.

The scientists involved in the project claimed they are providing a template that someone else can follow for any species of shellfish, anywhere in the world.

This summer, researchers from the University of Rhode Island will conduct field trials of OY15—as well as two other strains they have been developing independently—at commercial and university hatcheries. If all goes well during field trials, oyster probiotics could hit the market in 2 to 3 years.

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